This invention relates to the manufacture of polyolefin tow for use in cigarette filters. In particular, this invention relates to an improved polyolefin filter tow and a method of making it.
It is known to produce polyolefin filter tow by stretching polyolefin film while heating it in order to orient its molecular structure in the stretching direction, slitting the film to fibrillate it, and then subjecting the fibrillated film to a crimping operation. Crimping the fibrillated film gives it more bulk, and makes it "fluff up" so that it is more like traditional cigarette filter materials, such as cellulose acetate. One such polyolefin filter tow and its manufacture are described in U.S. Pat. No. 3,880,173.
The oriented film is slit in the direction of orientation, because the film tends to "crack" in that direction, so that it is easy to slit, while in the transverse direction it becomes more difficult to slit. However, in some cases it becomes so easy to slit the film in the orientation direction that a slit once started may continue too far, perhaps even to the end of the film.
When fibrillated polyolefin film is made into filter tow, and cigarette filters are made from the tow, the tow has a certain "yield", defined as the pressure drop obtainable from a given weight of filter tow. Yield may by measured, for example, in millimeters of water per milligram (mm WG/mg). It is desirable to maximize the yield from a given weight of filter tow.
One way to maximize tow yield from fibrillated polyolefin film is to control the tendency of the oriented film to crack.
It would be desirable to be able to control the properties of polyolefin film so that it fibrillates as desired, thereby to improve the yield of filter tow produced therefrom.